A number of monitoring systems for spark ignited engines have been developed in an attempt to effectively detect engine misfiring before engine performance deteriorates significantly. It has been found that the functioning of the engine ignition system can be tested to indicate an abnormal engine condition, such as a fouled or defective spark plug, an improperly balanced engine or a defective engine component associated with a particular cylinder. Such testing methods have proven to be particularly effective for monitoring spark plug condition, since this condition directly affects the function of the ignition system.
Many ignition system monitoring units measure a voltage characteristic occurring at the secondary winding of an ignition coil, and some of these systems are invasive and must be manually attached to the secondary winding. The voltage signal amplitude at the secondary winding is much greater than that of the signal at the primary winding, and these systems must perform the difficult task of accurately monitoring the high secondary voltage characteristic without disrupting the normal operation of the ignition system. Secondary voltage monitoring systems are shown by U.S. Pat. Nos. 3,793,584 to L. N. Liebermann et al., 3,942,102 to K. L. Kuhn et al., 4,006,403 to M. Olsen et al., 4,558,280 to S. E. Koehl et al., and 4,547,734 to H-W Spaude.
U.S. Pat. No. 4,277,752 to W. Dinkelackey et al. discloses a device for testing the ignition system of a combustion engine which includes an adjustable load connected in the primary winding of the ignition coil. The load is progressively increased until an ignition slip or misfire is detected, and from this, ignition energy reserve can be calculated to provide a measure for the condition of the whole ignition system. Although this system works from the primary winding of the ignition coil, it is very intrusive and can function only by intentionally causing a misfire. Thus no indication is provided of the performance of the ignition system under actual operation conditions.
Systems have been developed for monitoring voltage characteristics at the primary winding of an ignition coil during actual engine and ignition system operating conditions, and systems of this type are disclosed by U.S. Pat. Nos. 4,684,896 to W. Weishaupt and 4,918,389 to R. Schleupen et al. These patented systems rely upon waveform characteristics which are only present in inductive type ignition systems and which are not found in capacitive discharge (CD) type ignition systems. The peak primary voltage of a capacitive discharge ignition system is fixed, while the peak primary voltage of an inductive type ignition system varies as a function of the peak firing voltage (secondary voltage). Also, spark duration cannot readily be determined from the primary voltage waveform for a capacitive discharge ignition system, while the spark duration is easily determined from the primary voltage waveform in an inductive ignition system.
The misfire detection system and method of the Schleupen patent relies upon the extraction of spark duration information from the voltage in the primary winding of an ignition coil and the comparison of this information with a reference voltage of a predetermined magnitude and duration. The testing method and apparatus of the Weishaupt patent relies upon the peak primary voltage of an ignition coil being a function of the peak firing voltage to calculate spark plug condition. Thus neither of these systems will operate with a capacitive discharge ignition system.